freebsd-update: re-edit files if merge conflict markers remain

freebsd-update will open ${EDITOR} if conflicts occur while merging
updates to config files. Inform the user if they've left conflict
markers behind, and go back to editing the file.

PR: 185546
PR: 229689
Reviewed by: delphij
Approved by: re (cperciva, expedited MFC)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37703

(cherry picked from commit ceb5f28ba5fcfa69de7410d2327d4a5abf2a421f)